layout: Regardless of restyle damage, always reflow when viewport changes (#37099)

This fixes an issue where a viewport change did not trigger a reflow,
when the restyle damage was was otherwise REPAINT. Viewport changes
mean changes to the initial containing block, which is one of the main
inputs to layout. This should trigger a reflow always.

Testing: Unfortunately, our testing infrastructure is not good enough
yet
to test changes to layout when resizing the `WebView`, so it is quite
difficult to write tests for this change.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

2 files changed, 21 insertions, 34 deletions

diff --git a/components/layout/flow/root.rs b/components/layout/flow/root.rs
index 8ad3671032e..fb9884a4f01 100644
--- a/components/layout/flow/root.rs
+++ b/components/layout/flow/root.rs
@@ -5,6 +5,8 @@
 use app_units::Au;
 use atomic_refcell::AtomicRef;
 use compositing_traits::display_list::AxesScrollSensitivity;
+use euclid::Rect;
+use euclid::default::Size2D as UntypedSize2D;
 use malloc_size_of_derive::MallocSizeOf;
 use script::layout_dom::ServoLayoutNode;
 use script_layout_interface::wrapper_traits::{
@@ -27,7 +29,7 @@ use crate::flow::inline::InlineItem;
 use crate::flow::{BlockContainer, BlockFormattingContext, BlockLevelBox};
 use crate::formatting_contexts::IndependentFormattingContext;
 use crate::fragment_tree::FragmentTree;
-use crate::geom::{LogicalVec2, PhysicalPoint, PhysicalRect, PhysicalSize};
+use crate::geom::{LogicalVec2, PhysicalRect, PhysicalSize};
 use crate::positioned::{AbsolutelyPositionedBox, PositioningContext};
 use crate::replaced::ReplacedContents;
 use crate::style_ext::{Display, DisplayGeneratingBox, DisplayInside};
@@ -348,7 +350,7 @@ impl BoxTree {
     pub fn layout(
         &self,
         layout_context: &LayoutContext,
-        viewport: euclid::Size2D<f32, CSSPixel>,
+        viewport: UntypedSize2D<Au>,
     ) -> FragmentTree {
         let style = layout_context
             .style_context
@@ -358,13 +360,8 @@ impl BoxTree {
 
         // FIXME: use the document’s mode:
         // https://drafts.csswg.org/css-writing-modes/#principal-flow
-        let physical_containing_block = PhysicalRect::new(
-            PhysicalPoint::zero(),
-            PhysicalSize::new(
-                Au::from_f32_px(viewport.width),
-                Au::from_f32_px(viewport.height),
-            ),
-        );
+        let physical_containing_block: Rect<Au, CSSPixel> =
+            PhysicalSize::from_untyped(viewport).into();
         let initial_containing_block = DefiniteContainingBlock {
             size: LogicalVec2 {
                 inline: physical_containing_block.size.width,
diff --git a/components/layout/layout_impl.rs b/components/layout/layout_impl.rs
index 8162ed1dd0b..4ab69976671 100644
--- a/components/layout/layout_impl.rs
+++ b/components/layout/layout_impl.rs
@@ -16,7 +16,7 @@ use base::id::{PipelineId, WebViewId};
 use compositing_traits::CrossProcessCompositorApi;
 use constellation_traits::ScrollState;
 use embedder_traits::{UntrustedNodeAddress, ViewportDetails};
-use euclid::default::{Point2D as UntypedPoint2D, Rect as UntypedRect, Size2D as UntypedSize2D};
+use euclid::default::{Point2D as UntypedPoint2D, Rect as UntypedRect};
 use euclid::{Point2D, Scale, Size2D, Vector2D};
 use fnv::FnvHashMap;
 use fonts::{FontContext, FontContextWebFontMethods};
@@ -150,10 +150,6 @@ pub struct LayoutThread {
     /// A counter for epoch messages
     epoch: Cell<Epoch>,
 
-    /// The size of the viewport. This may be different from the size of the screen due to viewport
-    /// constraints.
-    viewport_size: UntypedSize2D<Au>,
-
     /// Scroll offsets of nodes that scroll.
     scroll_offsets: RefCell<HashMap<ExternalScrollId, Vector2D<f32, LayoutPixel>>>,
 
@@ -527,10 +523,6 @@ impl LayoutThread {
             stacking_context_tree: Default::default(),
             // Epoch starts at 1 because of the initial display list for epoch 0 that we send to WR
             epoch: Cell::new(Epoch(1)),
-            viewport_size: Size2D::new(
-                Au::from_f32_px(config.viewport_details.size.width),
-                Au::from_f32_px(config.viewport_details.size.height),
-            ),
             compositor_api: config.compositor_api,
             scroll_offsets: Default::default(),
             stylist: Stylist::new(device, QuirksMode::NoQuirks),
@@ -611,7 +603,8 @@ impl LayoutThread {
             ua_or_user: &ua_or_user_guard,
         };
 
-        if self.update_device_if_necessary(&reflow_request, &guards) {
+        let viewport_changed = self.viewport_did_change(reflow_request.viewport_details);
+        if self.update_device_if_necessary(&reflow_request, viewport_changed, &guards) {
             if let Some(mut data) = root_element.mutate_data() {
                 data.hint.insert(RestyleHint::recascade_subtree());
             }
@@ -658,6 +651,7 @@ impl LayoutThread {
             root_element,
             rayon_pool,
             &mut layout_context,
+            viewport_changed,
         );
 
         self.build_stacking_context_tree(&reflow_request, did_reflow);
@@ -683,12 +677,12 @@ impl LayoutThread {
     fn update_device_if_necessary(
         &mut self,
         reflow_request: &ReflowRequest,
+        viewport_changed: bool,
         guards: &StylesheetGuards,
     ) -> bool {
         let had_used_viewport_units = self.stylist.device().used_viewport_units();
-        let viewport_size_changed = self.viewport_did_change(reflow_request.viewport_details);
         let theme_changed = self.theme_did_change(reflow_request.theme);
-        if !viewport_size_changed && !theme_changed {
+        if !viewport_changed && !theme_changed {
             return false;
         }
         self.update_device(
@@ -696,7 +690,7 @@ impl LayoutThread {
             reflow_request.theme,
             guards,
         );
-        (viewport_size_changed && had_used_viewport_units) || theme_changed
+        (viewport_changed && had_used_viewport_units) || theme_changed
     }
 
     fn prepare_stylist_for_reflow<'dom>(
@@ -749,6 +743,7 @@ impl LayoutThread {
         root_element: ServoLayoutElement<'_>,
         rayon_pool: Option<&ThreadPool>,
         layout_context: &mut LayoutContext<'_>,
+        viewport_changed: bool,
     ) -> bool {
         let dirty_root = unsafe {
             ServoLayoutNode::new(&reflow_request.dirty_root.unwrap())
@@ -773,7 +768,7 @@ impl LayoutThread {
 
         let root_node = root_element.as_node();
         let damage = compute_damage_and_repair_style(layout_context.shared_context(), root_node);
-        if damage.is_empty() || damage == RestyleDamage::REPAINT {
+        if !viewport_changed && (damage.is_empty() || damage == RestyleDamage::REPAINT) {
             layout_context.style_context.stylist.rule_tree().maybe_gc();
             return false;
         }
@@ -794,10 +789,7 @@ impl LayoutThread {
             build_box_tree()
         };
 
-        let viewport_size = Size2D::new(
-            self.viewport_size.width.to_f32_px(),
-            self.viewport_size.height.to_f32_px(),
-        );
+        let viewport_size = self.stylist.device().au_viewport_size();
         let run_layout = || {
             box_tree
                 .as_ref()
@@ -852,10 +844,11 @@ impl LayoutThread {
             return;
         }
 
-        let viewport_size = LayoutSize::from_untyped(Size2D::new(
-            self.viewport_size.width.to_f32_px(),
-            self.viewport_size.height.to_f32_px(),
-        ));
+        let viewport_size = self.stylist.device().au_viewport_size();
+        let viewport_size = LayoutSize::new(
+            viewport_size.width.to_f32_px(),
+            viewport_size.height.to_f32_px(),
+        );
 
         // Build the StackingContextTree. This turns the `FragmentTree` into a
         // tree of fragments in CSS painting order and also creates all
@@ -949,9 +942,6 @@ impl LayoutThread {
             Au::from_f32_px(viewport_details.size.height),
         );
 
-        // TODO: eliminate self.viewport_size in favour of using self.device.au_viewport_size()
-        self.viewport_size = new_viewport_size;
-
         let device = self.stylist.device();
         let size_did_change = device.au_viewport_size() != new_viewport_size;
         let pixel_ratio_did_change = device.device_pixel_ratio().get() != new_pixel_ratio;